The Open Mobile Alliance (OMA) is an organization that works on standardization of inter-operability between individual mobile solutions. The OMA has specified two profiles in a broadcasting protocol proper for use in a broadcasting environment. One is a Digital Rights Management (DRM) profile and the other is a smart card profile. In the smart card profile, a video/audio stream encrypted by a specific key is transmitted to a mobile equipment through a broadcasting network. Also, a data stream including a key required for decrypting the encrypted video/audio stream is transmitted to a mobile equipment through the same broadcasting network, and is a Short-Term Key Message (STKM).
In the smart card profile, an encrypted Traffic Encryption Key (TEK) included in the STKM is not decrypted in the mobile equipment, but a received TEK is transmitted to a universal subscriber identity module (USIM), that is, a smart card. In the future, the TEK will be used to actually decrypt an encrypted service and content.
In the smart profile, a TEK to be used in the next period needs to be acquired in advance in order to secure stability from factors interfering with TEK acquisition such as a communication network trouble and a message processing delay. Accordingly, in the smart profile, a plurality of TEKs are inserted into an encrypted data field of a KEMAC (key data transport payload) in one MIKEY structure and are transmitted to the USIM.
When an STKM of the MIKEY structure including the plurality of TEKs is received, the USIM analyzes and decrypts the received STKM, generates a response message as illustrated in FIG. 1A, and transmits the generated message to the mobile equipment.
FIGS. 1A and 1B illustrate a structure of a response message according to STKM reception in a USIM, in which FIG. 1A illustrates a schematic structure of the response message and FIG. 1B illustrates a detailed structure of a data object field of the response message.
Referring to FIG. 1A, the data object field of the response message generated in the USIM has the structure as illustrated in FIG. 1B. The USIM inserts a plurality of TEK data decrypted in the data object field into the response message, and transmits the response message to the mobile equipment.
Conventionally, the mobile equipment inserts a plurality of key data into one MIKEY message and transmits the message to the USIM. However, there are the following problems when the USIM transmits a response message for MIKEY reception to the mobile equipment.
First, when the plurality of key data are included and transmitted in a key data sub-payload field of the MIKEY message, a TEK and a SALT key are transmitted together. The SALT key is included optionally. If needed, only the TEK may be included and transmitted. When the TEK and the SALT key are transmitted together or only the TEK data is transmitted, the USIM may not determine which of multiple TEKs corresponds to the SALT key.
In a response message for the STKM that the USIM has received from the mobile equipment, there is no structure having information about the number of key data inserted by the mobile equipment into the STKM or information for mapping a plurality of TEKs to TEK IDs.
Accordingly, there is a problem in that a TEK for decrypting video/audio data corresponding to the next period in a current stream may not be identified since it is impossible to identify a specific TEK as well as the number of TEKs inserted into the STKM when the mobile equipment receives a corresponding response message. There is another problem in that it is difficult to identify a TEK corresponding to a SALT key transmitted along with the TEK.